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Register a new userA search for diffuse bands in fullerene planetary nebulae: evidence of diffuse circumstellar bands
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Jos\\'e Jairo D\\'iaz jdiaz
Publication date
2014
fields
Physics
and research's language is
English
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Large fullerenes and fullerene-based molecules have been proposed as carriers of diffuse interstellar bands (DIBs). The recent detection of the most common fullerenes (C60 and C70) around some planetary nebulae (PNe) now enable us to study the DIBs towards fullerene-rich space environments. We search DIBs in the optical spectra towards three fullerene-containing PNe (Tc 1, M 1-20, and IC 418). Special attention is given to DIBs which are found to be unusually intense towards these fullerene sources. In particular, an unusually strong 4428A absorption feature is a common charateristic of fullerene PNe. Similar to Tc 1, the strongest optical bands of neutral C60 are not detected towards IC 418. Our high-quality (S/N > 300) spectra for PN Tc 1, together with its large radial velocity, permit us to search for the presence of diffuse bands of circumstellar origin, which we refer to as diffuse circumstellar bands (DCBs). We report the first tentative detection of two DCBs at 4428 and 5780 A in the fullerene-rich circumstellar environment around the PN Tc 1. Laboratory and theoretical studies of fullerenes in their multifarious manifestations (carbon onions, fullerene clusters, or even complex species formed by fullerenes and other molecules like PAHs or metals) may help solve the mystery of some of the diffuse band carriers.
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Recently, the presence of fullerenes in the interstellar medium (ISM) has been confirmed especially with the first confirmed identification of two strong diffuse interstellar bands (DIBs) with C60+. This justifies reassesing the importance of interstellar fullerenes of various sizes with endohedral or exohedral inclusions and heterofullerenes (EEHFs). The phenomenology of fullerenes is complex. In addition to fullerene formation in shock shattering, fully dehydrogenated PAHs in diffuse interstellar (IS) clouds could perhaps efficiently transform into fullerenes including EEHFs. But it is extremely difficult to assess their expected abundance, composition and size distribution, except for C60+. EEHFs share many properties with C60, as regards stability, formation/destruction and chemical processes, and many basic spectral features. We address the interstellar importance of various EEHFs as possible DIB carriers. Specifically, we discuss IS properties and the contributions of fullerenes of various sizes and charge such as C60+, metallofullerenes, heterofullerenes, fulleranes, fullerene-PAH compounds, H2@C60. We conclude that the landscape of interstellar fullerenes is probably much richer than heretofore realized. EEHFs, together with pure fullerenes of various sizes, have properties necessary to be suitably carriers of DIBs: carbonaceous nature; stability and resilience in the ISM; various heteroatoms and ionization states; relatively easy formation; few stable isomers; right spectral range; energy internal conversion; Jahn-Teller fine structure. This is supported by the C60+ DIBs. But, the lack of information about optical spectra other than C60 and IS abundances still precludes definitive assessment of the importance of fullerenes as DIB carriers. Their compounds could significantly contribute to DIBs, but it still seems difficult that they are the only important DIB carriers.
We present the first sample of diffuse interstellar bands (DIBs) in the nearby galaxy M33. Studying DIBs in other galaxies allows the behaviour of the carriers to be examined under interstellar conditions which can be quite different from those of the Milky Way, and to determine which DIB properties can be used as reliable probes of extragalactic interstellar media. Multi-object spectroscopy of 43 stars in M33 has been performed using Keck/DEIMOS. The stellar spectral types were determined and combined with literature photometry to determine the M33 reddenings E(B-V)_M33. Equivalent widths or upper limits have been measured for the {lambda}5780 DIB towards each star. DIBs were detected towards 20 stars, demonstrating that their carriers are abundant in M33. The relationship with reddening is found to be at the upper end of the range observed in the Milky Way. The line of sight towards one star has an unusually strong ratio of DIB equivalent width to E(B-V)_M33, and a total of seven DIBs were detected towards this star.
The identification of the carriers of the diffuse interstellar bands (DIBs) remains to be established, with the exception of five bands attributed to C60+, although it is generally agreed that DIB carriers should be large carbon-based molecules (with ~10-100 atoms) in the gas phase, such as polycyclic aromatic hydrocarbons (PAHs), long carbon chains or fullerenes. More specific possible carriers among PAHs are investigated, namely elongated molecules, which could explain a correlation between the DIB wavelength and the apparent UV resilience of their carriers. We address the case of polyacenes, C4N+2-H2N+4, with N~10-18 fused rectilinear aligned hexagons. Polyacenes are attractive DIB carrier candidates because their high symmetry and large linear size allow them to form regular series of bands in the visible range with strengths larger than most other PAHs, as confirmed by recent laboratory results up to undecacene (C46H26). Those with very strong bands in the DIB spectral domain are just at the limit of stability against UV photodissociation. They are part of the prominent PAH family of interstellar carbon compounds, meaning that only ~10-5 of the total PAH abundance is enough to account for a medium-strength DIB. After summarizing the current knowledge about the properties of polyacenes and recent laboratory results, the likelihood that they might meet the criteria for being carriers of some DIBs is addressed by reviewing the following properties: wavelength and strength of their series of visible bands; interstellar stability and abundances, charge state and hydrogenation; and DIB rotation profiles. No definite inconsistency has been identified that precludes polyacenes from being the carriers of some DIBs with medium or weak strength, including the so-called C2 DIBs. But additional experimental data about long acenes and their visible bands are needed to make robust conclusions
We present spectroscopic VLT/UVES observations of two emerging supernovae, the Type Ia SN 2001el and the Type II SN 2003hn, in the spiral galaxy NGC 1448. Our high resolution and high signal-to-noise spectra display atomic lines of Ca II, Na I, Ti II and K I in the host galaxy. In the line of sight towards SN 2001el, we also detect over a dozen diffuse interstellar bands (DIBs) within NGC 1448. These DIBs have strengths comparable to low reddening galactic lines of sight, albeit with some variations. In particular, a good match is found with the line of sight towards the sigma type diffuse cloud (HD 144217). The DIBs towards SN 2003hn are significantly weaker, and this line of sight has also lower sodium column density. The DIB central velocities show that the DIBs towards SN 2001el are closely related to the strongest interstellar Ca II and Na I components, indicating that the DIBs are preferentially produced in the same cloud. The ratio of the 5797 and 5780 DIB strengths (r~0.14) suggests a rather high UV field in the DIB environment towards SN 2001el. We also note that the extinction estimates obtained from the sodium lines using multiple line fitting agree with reddening estimates based on the colors of the Type Ia SN 2001el.
This paper considers a very special set of a few interstellar features --- broad diffuse interstellar bands (DIBs) at 4430, 4882, 5450, 5779 and 6175 AAAA. The set is small, and measurements of equivalent widths of these DIBs are challenging because of severe stellar, interstellar, and sometimes, also telluric contaminations inside their broad profiles. Nevertheless, we demonstrate that they do correlate pretty tightly (DIBs 4882 and 5450 to a lesser extent though) with other narrower diffuse bands, as well as with the color excess E(B$-$V). The studied broad DIBs correlate well with both interstellar molecule CH and interstellar K{sc i}, i.e. it is hardly possible to verify whether the environments, facilitating the formation of very broad DIB carriers, are dominated by either molecular or atomic gas as both these species likely occupy the same volume.
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